The present invention relates to a gain control circuit of an amplifier for listening via a loudspeaker in an apparatus further including a microphone and its associated amplifier, more specifically with a view to suppressing the Larsen effect, the circuit essentially consisting of a negative feedback loop reducing the gain of the amplifier for listening via a loudspeaker when the signal coming from the microphone exceeds a predetermined threshold.
The invention also relates to the use of this gain control circuit in a loudspeaking telephone set. It is a known practice that in all systems comprising both a loudspeaker, fed by its amplifier, and a microphone, whose signal is applied more or less directly to the amplifier, there is a risk of oscillation caused by the Larsen effect due to the acoustic coupling between the loudspeaker and the microphone. This is the case, for example, in a loudspeaking telephone set, to which case the invention refers more particularly, but not exclusively.
In a loudspeaking telephone set it is necessary to provide means for suppressing the state of oscillation due to the Larsen effect and it is highly desirable that this suppression occurs automatically by a considerable reduction of the gain of the loudspeaker amplifier, that is to say without the user of the telephone set having to carry out any operation.
Various technical solutions have already been proposed. Generally speaking, it can be stated that these solutions consist in providing on the one hand a circuit for detecting the state of oscillation due to the Larsen effect, supplying at the output a control signal which can be of the "all or nothing" type, or the proportional type, and on the other hand a so-called control circuit which influences the variable gain preamplifier stage and causes a reduction, or else a total cancelling of the gain when the circuit for detecting the Larsen effect has established that a state of oscillation exists.
An embodiment of a loudspeaking telephone set and gain control is represented in the French Patent Application FR-A-No. 2 537 810.
Essentially, however, such an arrangement is still subject to relaxation because, after an oscillation, a first gain reduction can make this oscillation disappear, whereas a moment later the gain control will increase again so that a second oscillation occurs, etc.
It will be readily understood that there is no simple solution to realize the desired function, and the more so as the circuit for detecting the Larsen effect must be very selective in distinguishing an oscillation signal from a high amplitude but temporary audio signal such as an ambient noise or an impact on the microphone.